Focusing type distance measurement apparatus

ABSTRACT

By providing a diffusing plate between a laser diode and an objective lens, a measurement light flux emitted from the laser diode is irradiated on a corner cube in a defocused state via the objective lens. The light flux having been reflected at corner cube sustains the defocused state. The reflected light flux having been reflected at and having exited the corner cube becomes condensed onto an avalanche photodiode through the objective lens. Even when a vibration occurs at the corner cube, the reflected light flux is received at the avalanche photodiode as long as the light flux having been reflected at and having exited the corner cube is within the predetermined range.

INCORPORATION BY REFERENCE

The disclosure of the following priority application is incorporatedherein by reference: Japanese Patent Application No. 2001-237662 filedAug. 6, 2001

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a focusing type distance measurementapparatus that measures the distance to a target object by transmittinglight so as to focus the light at the target object and then receivingthe light reflected or scattered from the target object.

2. Description of the Related Art

There are apparatuses that measure distances by using light waves in therelated art. In a distance measurement achieved by using light waves,the distance to a target object is determined by transmitting modulatedlight toward the target object, receiving the light reflected from thetarget object and detecting the difference between the phases of themodulation signals corresponding to the transmitted light and thereceived light. A distance measurement apparatus adopting thismeasurement method may measure the distance to a corner cube (prism)used as the target object by transmitting modulated light toward thecorner cube and receiving the light reflected from the corner cube or itmay instead measure the distance to the target object which is not acorner cube by causing modulated light to scatter at the surface of thetarget object and receiving the scattered light. An apparatus thatmeasures distances without using a corner cube is referred to as anon-prism type distance measurement apparatus. When light scattered atthe target object is received, the light quantity of the received lightflux is very much smaller than the light quantity of the received lightflux of the light reflected at a corner cube (approximately 1/1million). For this reason, a focusing type distance measurementapparatus adopts a configuration that allows measurement light(modulated light) to exit an objective lens toward the target object soas to condense the measurement light flux onto the target object andalso allows the scattered light from the target object to be condensedon a light-receiving element via the objective lens, to ensure that asufficient quantity of light is received. There is a demand for thistype of focusing type distance measurement apparatus that also allowsthe use of a corner cube as the target object.

If the position of the corner cube used as the target object becomesoffset or the corner cube vibrates, a collimation position of thefocusing type distance measurement apparatus becomes offset from thecenter of the corner cube. If the measurement light flux enters thecorner cube at a position offset from the center, the reflected lightexits the corner cube from the position symmetrical to the point ofentry at which the measurement light flux entered, relative to thecenter of the corner cube. This reflected light is transmitted throughthe objective lens as a light flux originating from a position off thecollimation position of the focusing type distance measurementapparatus, i.e., off the optical axis, and forms an image at a positionoff the optical axis. Since the light-receiving element is normallyprovided on the optical axis, the quantity of light entering thelight-receiving element changes as the corner cube vibrates, causingdifficulty in achieving an accurate distance measurement. Such avibration of the corner cube is particularly problematic in a shortdistance measurement, i.e., when the distance to the target object issmall.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a focusing typedistance measurement apparatus that facilitates measurement of a shortdistance (several tens of meters or less) performed by using a cornercube.

A focusing type distance measurement apparatus according to a firstaspect of the present invention comprises a collimating optical systemthat collimates the light from a target object, a light-transmittingoptical system that transmits a measurement light flux to the targetobject, a focusing optical system that condenses the measurement lightflux onto the target object, a light-receiving optical system thatreceives a reflected light flux reflected from the target object via thefocusing optical system and a deflecting optical member that changes thestate in which the measurement light flux is condensed on the targetobject.

The focusing type distance measurement apparatus in the first aspect ofthe present invention may further include an operating member thatoutputs an operation signal for performing distance measurement by usinga prism as the target object and a control circuit that implementscontrol on the deflecting optical member to achieve a first state at thedeflecting optical member so as to change the state in which themeasurement light flux is condensed when the following conditions arepresent; the operation signal has been output through the operatingmember and the light reception level indicating the quantity of thereflected light flux received at the light-receiving optical system isequal to or lower than a predetermined value or the light receptionlevel manifests a significant change and implements control on thereflecting optical member to achieve a second state so as not to changethe state in which the measurement light flux is condensed when theconditions are not present.

A focusing type distance measurement apparatus in a second aspect of thepresent invention, achieved by replacing the deflecting optical memberthat changes the state in which the measurement light flux is condensedon the target object with a deflecting light optical member that changesthe state in which the reflected light flux received at thelight-receiving optical system is condensed, further includes anoperating member that outputs an operation signal for performingdistance measurement by using a prism as the target object and a controlcircuit that implements control on the deflecting optical member achievea first state so as to change the state in which the measurement lightflux is condensed when the following conditions are present; theoperation signal has been output through the operating member and thelight reception level indicating the quantity of the reflected lightflux received at the light-receiving optical system is equal to or lowerthan a predetermined value or the light reception level manifests asignificant change and implements control on the deflecting opticalmember to achieve a second state so as not to change the state in whichthe measurement light flux is condensed when the conditions are notpresent.

A focusing type distance measurement apparatus in a third aspect of thepresent invention, achieved by replacing the deflecting optical memberthat changes the state in which the measurement light flux is condensedon the target object with a deflecting optical member that changes thestate in which a reflected light flux received at the light-receivingoptical system is condensed, further includes a control circuit thatimplements control on the deflecting optical member so as not to changethe state in which the reflected light flux is condensed if the lightreception level indicating the quantity of the reflected light fluxreceived at the light-receiving optical system exceeds a predeterminedvalue.

A focusing type distance measurement apparatus in a fourth aspect of thepresent invention, achieved by replacing the deflecting optical memberthat changes the state in which the measurement light flux is condensedon the target object with a deflecting optical member that changes thestate in which a reflected light flux received at the light-receivingoptical system is condensed, further includes an operating member thatoutputs an operation signal for performing a short distance measurementby using a prism as the target object and a control circuit thatimplements control on the deflecting optical member to achieve a firststate so as to change the state in which the measurement light flux iscondensed when the operation signal has been output through theoperating member and implements control on the deflecting optical memberto achieve a second state so as not to change the state in which themeasurement of light flux is condensed when the operation signal has notbeen output.

A focusing type distance measurement apparatus in a fifth aspect of thepresent invention comprises a collimating optical system that collimatesthe light from a target object, a light-transmitting optical system thattransmits a measurement light flux to the target object, a focusingoptical system that condenses the measurement light flux onto the targetobject, a light-receiving optical system that receives a reflected lightflux reflected from the target object via the focusing optical system,an internal light path through which the light flux transmitted from alight source at the light-transmitting optical system is guided to alight-receiving unit in the light-receiving optical system within themeasurement apparatus instead of allowing the light flux to betransmitted toward the target object, an external light path throughwhich the measurement light flux transmitted from the light-transmittingoptical system is guided to the target object and the reflected lightflux reflected from the target object is guided to the light-receivingoptical system and a switching shutter that selects either the internallight path or the external light path for the light flux transmittedfrom the light source to be guided through, with a deflecting opticalmember that changes the state in which the measurement light flux iscondensed on the target object provided at the switching shutter.

The focusing type distance measurement apparatus in the fifth aspect ofthe present invention may further include a first operating member thatoutputs an operation signal to enable distance measurement by using aprism as the target object and a control circuit that implements controlon the switching shutter so as to change the state in which themeasurement light flux is condensed when the operation signal has beenoutput through the first operating member and the light reception levelindicating the quantity of the reflected light flux received at thelight-receiving optical system is equal to or lower than a predeterminedvalue or the light reception level manifests a significant change.

Alternatively, the focusing type distance measurement apparatus in thefifth aspect of the present invention may further include a secondoperating member that outputs an operation signal for performing a shortdistance measurement by using a prism as the target object and a controlcircuit that implements control on the switching shutter so as to changethe state in which the measurement light flux is condensed when theoperation signal has been output from the second operating member.

In the focusing type distance measurement apparatuses described above, alight attenuator that adjusts the light reception level of the lightreceived at the light-receiving optical system may be provided at thedeflecting optical member. It is desirable to adjust the light receptionlevel through the light attenuator when the deflecting optical member isin the second state. Such a deflecting optical member may include aplurality of light transmittance adjustment areas provided along thecircumference of a rotating plate and a diffusion area where the lightflux is diffused provided along the direction in which the plurality oflight transmittance adjustment areas are arranged. In this case, thecontrol circuit may include a motor in order to rotate the rotatingplate. In addition, the control circuit inserts the diffusion area at ameasurement light path when the deflecting optical member is in thefirst state and inserts one of the plurality of light transmittanceadjustment areas in the measurement light path when the deflectingoptical member is in the second state.

The deflecting optical member in a focusing type distance measurementapparatus according to the present invention may be constituted of anoptical member that achieves a diffusing function or an optical memberthat achieves a refracting function.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic configuration of the optical systems in afocusing type distance measurement apparatus;

FIG. 2 illustrates the light-transmitting optical system in the focusingtype distance measurement apparatus achieved in a first embodiment;

FIG. 3 shows the optical systems including the light-receiving opticalsystem;

FIG. 4 illustrates the misalignment of the corner cube center relativeto the optical axis and the misalignment of the received light fluxrelative to the optical axis, manifesting when a parallel light fluxenters the objective lens;

FIG. 5 illustrates the ND filter;

FIG. 6 presents a flowchart of the distance measurement processingexecuted by the control circuit;

FIG. 7 shows an example of a variation of the light-transmitting opticalsystem;

FIG. 8 presents a flowchart of the distance measurement processingexecuted by the control circuit in a second embodiment;

FIG. 9 illustrates the light-receiving optical system in the focusingtype distance measurement apparatus achieved in a third embodiment;

FIG. 10 shows an arrangement in which the deflecting plate ispermanently secured;

FIG. 11 presents an example of a variation of the light-receivingoptical system;

FIG. 12 presents yet another example of the light-receiving opticalsystem;

FIG. 13 shows the optical systems in the focusing type distancemeasurement apparatus achieved in a fourth embodiment;

FIG. 14 shows the switching shutter achieved in the fourth embodiment;and

FIG. 15 shows the switching shutter achieved in the fourth embodimentand the positions of the light flux assumed therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is an explanation of the embodiments of the presentinvention, given in reference to the drawings.

First Embodiment

FIG. 1 illustrates the basic configuration of the optical systems in afocusing type distance measurement apparatus. In FIG. 1, a pulse currentis supplied from a drive circuit (not shown) to an LD (laser diode) 7and, as a result, the LD 7 emits pulse-modulated measurement light. Thepulse light emitted from the LD 7 enters a dichroic prism 3 via asplitting prism 6. The dichroic prism 3 achieves characteristics wherebyit reflects infrared light from the LD 7 and allows light having a wavelength in the visible light range to be transmitted. The pulse lighthaving entered the dichroic prism 3 is reflected inside the dichroicprism 3 and is then emitted toward a target object 9 via a focusing lens2 and an objective lens 1.

The reflected light (scattered light) from the target object 9attributable to the measurement light from the LD 7 enters the dichroicprism 3 via the objective lens 1 and the focusing lens 2. The scatteredlight having entered the dichroic prism 3 is reflected inside thedichroic prism 3, and then enters an APD (avalanche photodiode) 8 viathe splitting prism 6. The reflected light (scattered light) from thetarget object 9 attributable to the illuminating light (e.g., naturallight) irradiating the target object 9, on the other hand, enters thedichroic prism 3 via the objective lens 1 and the focusing lens 2, andits visible light component passes through the dichroic prism 3 to forman image of the target object 9 at a focusing plate 4. The image of thetarget object 9 formed on the focusing plate 4 is observed by the userthrough an eyepiece lens 5.

In FIG. 1, the LD 7, the APD 8 and the focusing plate 4 are set atoptically conjugate positions. Thus, when the focusing lens 2 is drivenalong the optical axis in the horizontal direction in FIG. 1 so that thefocal point of the focusing lens 2 is on the target object 9, themeasurement light flux is condensed on the target object 9. At thistime, the reflected light (scattered light) from the target object 9,too, is condensed at the APD 8. By adopting the configuration describedabove, the light-transmitting optical system, the light-receivingoptical system and the collimating optical system of the focusing typedistance measurement apparatus are achieved.

FIG. 2 illustrates the light-transmitting optical system in the focusingtype distance measurement apparatus achieved in the first embodiment ofthe present invention. A corner cube 10 is used as the target object ofthe distance measurement. The corner cube 10 is constituted of a prismthat includes reflecting surfaces perpendicular to each other as shownin the figure and radiates reflected light parallel to the incidentlight. FIG. 2 does not include the illustration of the focusing lens,the splitting prism, the dichroic prism, the collimating optical system(the focusing plate and the eyepiece lens) and the like. A deflectingplate 12 achieving a diffusing function is provided between the LD 7 andthe objective lens 1. If there is no deflecting plate 12, themeasurement light flux emitted from the LD 7 will be condensed at thecenter of the corner cube 10, i.e., the target object, as indicated withthe dotted line in FIG. 2. However, since the deflecting plate 12 isprovided in reality, the measurement light flux exiting the deflectingplate 12 advances toward the corner cube 10 from a new light sourceposition, i.e., from the deflecting plate 12, as indicated with thesolid line in FIG. 2, to be irradiated onto the corner cube 10 as abroader light flux. In other words, the measurement light flux becomesdefocused on the corner cube 10.

FIG. 3 shows the optical systems including the light receiving opticalsystem which receives the reflected light resulting from the light castby the light-transmitting optical system shown in FIG. 2. While theactual reflected light is turned back at the surface of the corner cube10, the reflected light shown in FIG. 3 is not turned back for purposesof simplicity. In other words, objective lenses 1 and 1′ are a singlelens in reality.

In FIG. 3, the measurement light flux emitted from the LD 7 isirradiated in a defocused state onto the corner cube 10 via theobjective lens 1 as indicated with the solid line. Since the corner cube10 constitutes the target object, the defocused state of the measurementlight flux is sustained after the measurement light flux is reflected,and the defocused reflected light flux exits the corner cube 10. Thisreflected light flux is condensed on the APD 8 by the objective lens 1′.It is to be noted that the position of the focusing lens which is notshown in FIG. 3 is adjusted at the position at which a focal point ofthe collimating optical system is on the target object.

Due to the presence of the deflecting plate 12, the light transmittingoptical system and the light-receiving optical system are no longerconjugate with each other and, as a result, the reflected light flux(the received light flux) is not focused at the light-receiving surfaceof the APD 8. However, since the APD 8 only needs to output aphotocurrent corresponding to the quantity of light entering itslight-receiving surface, it does not matter if the image is not focusedat the light-receiving surface as long as the reflected light flux atleast enters the light-receiving surface. FIG. 3 shows that thereflected light can be received at the APD 8 as long as the reflectedlight flux exiting the corner cube 10 is within the range h. Theembodiment of the present invention is characterized in that when thecorner cube 10 is used as the target object in the distance measurement,the deflecting plate 12 is inserted between the LD 7 and the objectivelens 1 to allow the reflected light flux to enter the APD 8 with ease.

A structure which does not include the deflecting plate 12 is nowexplained in reference to FIG. 3. The measurement light flux emittedfrom the LD 7 is irradiated via the objective lens 1 to achieve afocused state at the center of the corner cube 10 as indicated with thedotted line. The reflected light flux advances toward the objective lens1′ from the new light source position, i.e., from the center of thecorner cube 10, and forms an image on the APD 8 through the objectivelens 1′. In this case, the reflected light flux is received at the APD 8only if the center of the corner cube 10 is on the optical axis AX or ifthe center of the corner cube 10 is in close proximity to the opticalaxis AX. Namely, in the case of the light flux indicated with the dottedline in FIG. 3, the reflected light flux cannot be readily condensed onthe APD 8 once the center of the corner cube 10 becomes offset from theoptical axis AX.

FIG. 4 illustrates the misalignment of the center of the corner cube 10from the optical axis AX and the misalignment of the received light fluxfrom the optical axis AX, manifesting when the parallel light fluxenters the objective lens 1′. In FIG. 4, D represents the distance fromthe corner cube 10 to the objective lens 1′, L represents the distancefrom the objective lens 1′ to the position at which a focused image isformed with the received light flux (reflected light flux), S representsthe distance (the shift quantity) between the optical axis AX and theposition at which the focused image is formed with the reflected lightflux, attributable to the collimation shift of the corner cube 10 and Yrepresents the distance between the optical axis AX and the position atwhich the focused image is formed with the received light flux. Thesedistances achieve a relationship expressed as in (1) below.

Y=(S×L)/D  (1)

When L=200 mm and S=10 mm with regard to the optical systems in thedistance measurement apparatus, the distance Y between the optical axisAX and the position at which the focused image is formed with thereceived light flux increases as indicated below in reverse proportionto the change in the distance D as the distance D between the cornercube 10 and the objective lens 1′ is reduced.

D L S Y 20 m 200 mm 10 mm 0.1 mm 10 m 200 mm 10 mm 0.2 mm  5 m 200 mm 10mm 0.4 mm  2 m 200 mm 10 mm 1.0 mm

The numerical values above indicate that when the distance D from thecorner cube 10 to the objective lens 1′ is equal to or less than 20 m,the distance Y between the optical axis AX and the position at which thefocused image is formed with the received light flux is equal to orgreater than 0.1 mm. The area over which light is received at the APD 8is normally within the range of several tens of μmø˜0.2 mmø. Thus, ifthe incident light flux becomes offset from the optical axis AX and Ybecomes equal to or greater than 0.1 mm, the received light flux nolonger enters the light-receiving surface of the APD 8 set on theoptical axis AX. In such a situation, the deflecting plate 12 isinserted between the LD 7 and the objective lens 1 and the defocusedmeasurement light flux is caused to enter the corner cube 10 so as toallow the reflected light flux to enter the APD 8. It is to be notedthat when the optical systems assume the numerical values indicatedabove, the distance Y from the optical axis AX to the position at whichthe focused image is formed with the received light flux is equal to orless than 0.1 mm if the distance D is equal to or greater than 20 m and,in such a case, it is not necessary to insert the deflecting plate 12.

In the focusing type distance measurement apparatus, the quantity oflight received at the APD 8 is significantly affected by factors such asthe distance D to the target object, the material constituting thetarget object and the type of surface treatment that the target objecthas undergone. Accordingly, a light variable attenuator is provided soas to adjust the quantity of light entering the APD 8 at a constantlevel. This light variable attenuator may be constituted by, forinstance, an ND filter or the like which is rotatably inserted in alight path to achieve a gradual change in the concentration of light.The light path at which the light variable attenuator is inserted islocated between the LD 7 and the objective lens 1.

FIG. 5 illustrates an ND filter. The ND filter is constituted by forminga band of varying concentration from a point P to a point Q around afilm formed in a round shape having a center O. The concentration overthe area corresponding to the point P is the highest, the concentrationgradually becomes lower along the clockwise direction and theconcentration over the area corresponding to the point Q is the lowest.A diffusing plate is pasted over the area corresponding to the point R.The ND filter is driven by a motor M to rotate around the center O. As aresult, the concentration of the portion inserted in the light pathchanges to adjust the degree to which the light is attenuated. The statein which the diffusing plate is left inserted in the light path as themotor M comes to a halt is equivalent to a state in which the deflectingplate 12 described above is set in place.

The motor M may be, for instance, a stepping motor, and its rotatingangle is controlled in conformance to a drive pulse signal provided by acontrol circuit 51. The control circuit 51 calculates the necessaryattenuation quantity based upon the photocurrent output from the APD 8,and generates a pulse signal for rotating the motor M so as to insertthe portion of the ND filter with the correct concentration forachieving the attenuation quantity into the light path. When thedeflecting plate 12 is required, the control circuit 51 generates apulse signal for rotating the motor M so as to insert the deflectingplate 12 into the light path.

Operation signals are individually input to the control circuit 51through a corner cube in-use button 52 and a corner cube not-in-usebutton 53. If an operation signal is input through the corner cubein-use button 52, the control circuit 51 judges that a prism mode iscurrently selected, whereas if an operation signal is input through thecorner cube not-in-use button 53, the control circuit 51 judges that anon-prism mode is currently selected. The prism mode is an operatingmode in which the distance is measured by using a corner cube as thetarget object, and the non-prism mode is an operating mode in whichdistance is measured without using the corner cube.

The distance measurement processing executed by the control circuit 51of the focusing type distance measurement apparatus described above isnow explained in reference to the flowchart presented in FIG. 6. In thefirst embodiment, the deflecting plate 12 is inserted between the LD 7and the objective lens 1 as necessary while the focusing type distancemeasurement apparatus is set in the prism mode. In step S11, the controlcircuit 51 makes a decision as to whether or not the prism mode iscurrently selected. If the distance measurement apparatus is set in theprism mode, an affirmative decision is made in step S11 and theoperation proceeds to step S12, whereas if the distance measurementapparatus is not set in the prism mode, a negative decision is made instep S11 and the operation proceeds to step S20.

In step S12, the control circuit 51 makes a decision as to whether ornot a measurement has been started. An affirmative decision is made instep S12 if an operation signal has been input through a start button(not shown) and in this case, the operation proceeds to step S13,whereas a negative decision is made in step S12 if no operation signalhas been input through the start button and the operation returns tostep S11. In step S13, the control circuit 51 outputs a command for theLD 7 to engage in a pulse light emission before the operation proceedsto step S14.

In step S14, the control circuit 51 makes a decision as to whether ornot it is necessary to insert the deflecting plate 12. The controlcircuit 51 makes an affirmative decision in step S14 if the value of thephotocurrent output by the APD 8 is smaller than a predetermined valueor the value of the photocurrent manifests a significant change (e.g., achange by a factor of 1000 or greater), and then the operation proceedsto step S15. If the value of the photocurrent is smaller than thepredetermined value, the light reception level is low. If, on the otherhand, the value of the photocurrent is equal to or greater than thepredetermined value and the photocurrent value does not show asignificant change, a negative decision is made in step S14 and theoperation proceeds to step S18. A significant change in the photocurrentvalue is caused when the corner cube 10 vibrates to become offset fromthe collimation position and thus the reflected light flux fails to becondensed on the APD 8 so that the light reception level changessignificantly.

In step S15, the control circuit 51 outputs a pulse signal for rotatingthe motor M so as to insert the deflecting plate 12 within the lightpath, before the operation proceeds to step S16. In step S16, thecontrol circuit 51 outputs a command for the LD 7 to engage in a pulselight emission, before the operation proceeds to step S17. In step S17,the control circuit 51 calculates the distance to the target objectbased upon the difference between the phase of the timing of thetransmitted pulse light and the phase of the timing of the photocurrentoutput from the APD 8, and then the processing in FIG. 6 ends.

Instep S18 to which the operation proceeds after making a negativedecision in step S14, the control circuit 51 calculates the attenuationquantity so as to set the value of the photocurrent output from the APD8 within a predetermined range and outputs a pulse signal for achievingthe calculated attenuation quantity to the motor M before the operationproceeds to step S19. In step S19, the control circuit 51 outputs acommand for the LD 7 to engage in a pulse light emission, and then theoperation proceeds to step S17.

In step S20, to which the operation proceeds if the distance measurementapparatus is not set in the prism mode, the control circuit 51implements non-prism mode measurement processing in the known art beforeending the processing in FIG. 6.

The level of the scattered light scattered at the surface of the targetobject 9 is normally as low as 1/1 million of the level of the reflectedlight from the corner cube 10. For this reason, the light-receivingsensitivity at the light-receiving optical system is set high for thenon-prism mode measurement in which the distance is measured byreceiving scattered light. If this light-receiving sensitivity settingis sustained during a prism mode measurement in which the distance ismeasured by receiving the reflected light from the corner cube 10, anexcessive quantity of light is received and thus, the signal levelbecomes too high. Accordingly, by making a negative decision in step S14to attenuate the received light flux, the quantities of light actuallyentering the APD 8 in the non-prism mode and in the prism mode areadjusted to levels substantially equal to each other. Since thereflected light flux becomes defocused on the APD 8 while the diffusingplate is inserted, the signal level at the APD 8 does not become toohigh.

The following advantages are achieved in the first embodiment describedabove.

(1) When the focusing type distance measurement apparatus, capable ofmeasuring a distance both in the non-prism mode, in which a distancemeasurement is performed by using a target object 9 other than thecorner cube 10, and in the prism mode, in which a distance measurementis executed by using the corner cube 10, is set in the prism mode, thedeflecting plate 12 is inserted between the LD 7 that emits themeasurement light and the objective lens 1 if the quantity of lightreceived at the APD 8 is smaller than a predetermined value or if thequantity of received light changes significantly (by a factor of atleast 1000, for instance). As a result, a defocused measurement lightflux enters the corner cube 10 and, consequently, the reflected lightflux can be condensed on the APD 8. This feature is particularlyadvantageous when the distance D between the objective lens 1 and thecorner cube 10 is small and there is vibration in the corner cube 10.

(2) If the quantity of light received at the APD 8 is greater than thepredetermined value, the concentration at the ND filter inserted in thelight path between the LD 7 and the objective lens 1 is changed toensure that a substantially constant quantity of light is received atthe APD 8. Since the photocurrent output from the APD 8 is adjusted toachieve a substantially constant value, the gain at the subsequent stagerelative to the APD 8 in the light-receiving circuit does not need to bechanged and thus, the same gain can be used even when the scatteredlight level changes. As a result, a cost reduction is achieved over astructure in which the gain must be variable.

(3) Since the deflecting plate 12 is pasted onto part of the round filmconstituting the ND filter, it is not necessary to add a special opticalsystem in order to insert the deflecting plate 12 into the light path.Thus, the apparatus can be achieved as a compact unit and can beproduced at reduced cost.

While the extent by which the light is attenuated through the ND filterdoes not change while the deflecting plate 12 is inserted in the lightpath in the explanation given above, the light attenuation quantityachieved through the filter may change while the deflecting plate 12 isinserted, instead. In the latter case, a plurality of deflecting platesshould be individually pasted onto portions with varying concentrationformed at the round film so that as a different deflecting plate isinserted in the light path, the attenuation quantity achieved throughthe filter changes.

In step S14 explained above, a decision is made by the control circuit51 as to whether or not it is necessary to insert the deflecting plate12 and the deflecting plate 12 is inserted into the light path if anaffirmative decision is made in step S14. Instead, a short distancemeasurement button (not shown) may be provided in the focusing typedistance measurement apparatus. In this focusing type distancemeasurement apparatus, a decision should be made as to whether or notthe short distance measurement button has been operated by the user andthe deflecting plate 12 should be inserted within the light path if theshort distance measurement button has been operated.

Instead of the deflecting plate 12 achieving a diffusing function, adeflecting plate achieving a refracting function may be utilized. FIG. 7shows an example of a variation of the light-transmitting optical systemachieved in the first embodiment. The illustration does not include thefocusing lens, the splitting prism, the dichroic prism, the collimatingoptical system (the focusing plate and the eyepiece lens) and the like.In FIG. 7, a deflecting plate 12A constituted of a concave lensachieving a refracting function is provided between the LD 7 and theobjective lens 1. A measurement light flux exiting the deflecting plate12A advances toward the corner cube 10 from a new light source position,i.e., from the deflecting plate 12A, as indicated with the solid line inFIG. 7, and is irradiated onto the corner cube 10 as a broader lightflux. Namely, the measurement light flux can achieve a defocused stateon the corner cube 10 when a concave lens is utilized as a deflectingplate as well.

Second Embodiment

In the second embodiment, the deflecting plate 12 inserted between theLD 7 and the objective lens 1 is withdrawn as necessary while thefocusing type distance measurement apparatus is set in the prism mode.The distance measurement processing executed by the control circuit 51of the focusing type distance measurement apparatus in the secondembodiment is now explained in reference to the flowchart presented inFIG. 8. In step S31, the control circuit 51 makes a decision as towhether or not the prism mode is currently selected. If the prism modeis currently selected, an affirmative decision is made in step S31 andthe operation proceeds to step S32, whereas if the prism mode is notcurrently set, a negative decision is made in step S31 and the operationproceeds to step S40.

In step S32, the control circuit 51 outputs a pulse signal for rotatingthe motor M so as to insert the deflecting plate 12 into the light pathbefore the operation proceeds to step S33. In step S33, the controlcircuit 51 makes a decision as to whether or not the measurement hasstarted. An affirmative decision is made in step S33 if an operationsignal has been input through the start button (not shown) and in thiscase, the operation proceeds to step S34, whereas a negative decision ismade in step S33 if no operation signal has been input through the startbutton and the operation returns to step S31.

In step S34, the control circuit 51 outputs a command for the LD 7 toengage in a pulse light emission before the operation proceeds to stepS35. In step S35, the control circuit 51 makes a decision as to whetheror not it is necessary to withdraw the deflecting plate 12. The controlcircuit 51 makes an affirmative decision in step S35 if the value of thephotocurrent output from the APD 8 is equal to or greater than apredetermined value and the photocurrent value has not changedsignificantly (e.g., by a factor of 1000 or more) and, in such a case,the operation proceeds to step S36. If, on the other hand, thephotocurrent value is smaller than the predetermined value or thephotocurrent value has changed significantly, a negative decision ismade in step S35 and the operation proceeds to step S39.

In step S36, the control circuit 51 outputs a pulse signal for rotatingthe motor M so as to withdraw the deflecting plate 12 from the lightpath and then the operation proceeds to step S 37. In step S37, thecontrol circuit 51 calculates the attenuation quantity so as to set thevalue of the photocurrent output from the APD 8 within a predeterminedrange and outputs a pulse signal for achieving the calculatedattenuation quantity to the motor M before the operation proceeds tostep S38. In step S38, the control circuit 51 outputs a command for theLD 7 to engage in a pulse light emission and then the operation proceedsto step S39.

In step S39, the control circuit 51 calculates the distance to thetarget object based upon the difference between the phase of the timingof the transmitted pulse light and the phase of the timing of thephotocurrent output from the APD 8, and then the processing in FIG. 8ends. In step S40, to which the operation proceeds if the distancemeasurement apparatus is not set in the prism mode, the control circuit51 implements a conventional non-prism mode measurement processingbefore ending the processing in FIG. 8.

Advantages similar to those realized in the first embodiment areachieved in the second embodiment described above.

Third Embodiment

The third embodiment is characterized in that when the corner cube 10 isused as the target object, a deflecting plate is inserted between theobjective lens 1 and the APD 8 to allow the reflected light flux toenter the APD 8 with ease. FIG. 9 illustrates the light-receivingoptical system in the focusing type distance measurement apparatusachieved in the third embodiment of the present invention. FIG. 9 doesnot include the illustration of the focusing lens, the splitting prism,the dichroic prism, the collimating optical system (the focusing plateand the eyepiece lens) and the like. It differs from the light-receivingoptical system in the first embodiment in that no deflecting plate 12 isprovided between the LD 7 which emits the measurement light and theobjective lens 1 and in that a deflecting plate 12B which achieves adiffusing function is provided between the objective lens 1′ and the APD8.

FIG. 9 shows that the center of the corner cube 10 is offset by adistance S/2 relative to the optical axis AX due to a vibration of thecorner cube or the like. When the measurement light flux enters thecorner cube at a position M distanced from the center of the corner cube10 by S/2, the reflected light exits from a position N which issymmetrical to the point of entry of the measurement light flux relativeto the center of the corner cube. This would result in the reflectedlight passing through the objective lens 1′ as a light flux originatingfrom a position distanced from the collimation position of the focusingtype distance measurement apparatus, i.e., from the optical axis AX, byS and forming an image at a position X distanced from the optical axisAX by Y.

However, since the deflecting plate 12B is provided, the reflected lightflux exiting the deflecting plate 12B will have been scattered at thedeflecting plate 12B and part of the scattered light enters thelight-receiving surface of the APD 8.

As explained earlier, the quantity of light received at the APD 8 of thefocusing type distance measurement apparatus is significantly affectedby factors such as the distance D to the target object, the materialconstituting the target object and the type of surface treatment thatthe target object has undergone. Accordingly, a light variableattenuator is provided in the optical path between the objective lens 1′and the APD 8 in order to adjust the quantity of light entering the APD8 at a constant level. As in the first embodiment, the deflecting plate12B is pasted onto part of the ND filter constituting the light variableattenuator. As the control circuit 51 outputs a pulse signal forrotating the motor M, the motor M starts to rotationally drive the roundND filter and, as a result, a portion of the ND filter with the desiredconcentration or the deflecting plate 12B at the ND filter becomesinserted at the light path.

A decision as to whether or not the deflecting plate 12B should beinserted at the light path should be made as in the decision-makingprocessing executed in the first and second embodiments to decidewhether or not the deflecting plate 12 is to be inserted.

In the third embodiment described above, instead of inserting thedeflecting plate 12 between the LD 7, which emits the measurement light,and the objective lens 1, the deflecting plate 12B is inserted betweenthe objective lens 1′ and the APD 8. Thus, the reflected light fluxhaving passed through the objective lens 1′ becomes scattered to allowsome of the reflected light flux offset from the optical axis AX to bereceived at the APD 8. This feature is particularly advantageous whenthe distance D between the objective lens 1 and the corner cube 10 issmall and there is a vibration in the corner cube 10.

FIG. 10 shows a deflecting plate 12C provided at a permanently fixedposition. It is desirable to ensure that this deflecting plate does notblock the light path on the optical axis AX through which the receivedlight flux attributable to the scattered light passes in the non-prismmode. Accordingly, the deflecting plate 12C assumes a shape achieved byconcentrically cutting out a part thereof around the optical axis AX soas to ensure that it does not block the light path of the scatteredlight indicated with the dotted line in FIG. 10. Since this deflectingplate 12C does not further attenuate the scattered light during anon-prism mode measurement, the quantity of the scattered light, whichis much less than the quantity of reflected light from a corner cube tobegin with, does not become reduced. It is to be noted that a focusingtype distance measurement apparatus adopting this structure includesanother ND filter which adjusts the quantity of the received light.

Instead of the deflecting plate 12B shown in FIG. 9, a deflecting plateachieving a refracting function may be utilized. FIG. 11 presents anexample of a variation of the light-receiving optical system achieved inthe third embodiment. FIG. 11 does not include the illustration of thefocusing lens, the splitting prism, the dichroic prism, the collimatingoptical system (the focusing plate and the eyepiece lens) and the like.A deflecting plate 12D constituted of a convex lens having a refractingfunction is provided between the objective lens 1′ and the APD 8. Thereflected light flux exiting the deflecting plate 12D advances towardthe light-receiving surface of the APD 8. Namely, the reflected lightflux can be received at the APD 8 when a convex lens is used as well.

Alternatively, a deflecting plate having a free-form surface may beutilized. In FIG. 12, a deflecting plate 12E having a reflectingfunction is provided between the objective lens 1′ and the APD 8. Thereflected light flux exiting the deflecting plate 12E advances towardthe light-receiving surface of the APD 8. Namely, the reflected lightflux can be received at the APD 8 when a deflecting plate is utilized aswell.

One of the various types of deflecting plates described above may beutilized alone or different types of deflecting plates may be utilizedin combination.

Any of the deflecting plates described above is inserted further towardthe LD 7 or the APD 8 rather than toward the dichroic prism 3 in FIG. 1.Thus, the deflecting plate inserted in the light-transmitting opticalsystem or the light-receiving optical system does not affect thecollimating optical system provided to enable an observation of an imageformed with visible light in any way whatsoever.

Fourth Embodiment

FIG. 13 shows the basic configuration of the optical systems adopted inthe fourth embodiment. This structure includes a switching shutter 20 inaddition to the optical systems shown in FIG. 1. The switching shutter20 selects either an external light path for the pulse light emittedfrom the LD 7 to be guided through to the target object or an internallight path.

The internal light path in this context refers to a light path providedto cancel out any measurement error attributable to changes in thecharacteristics of the devices such as the light-receiving device usedin the electronic circuit caused by a temperature change. In ahigh-precision distance measurement apparatus, the distance from thedistance measurement apparatus to the target object is determined bysubtracting the distance measured through the internal light path fromthe distance measured through the external light path.

In the embodiment, the switching shutter 20 is rotated to switch betweenthe internal light path and the external light path. FIG. 14 presents afront view of the switching shutter 20 (viewed along the optical axis)in the fourth embodiment. The switching shutter 20, which is formed in asubstantially round shape, includes an internal path window 21 providedat part of its outer edge. It also includes external path windows 22 and23 formed by cutting out portions of the shutter in a fan shape. Whilethe external path window 22 is a simple opening, an optical member 24achieving a diffusing function is provided at the other external lightpath window 23 and, as a result, the light flux of pulse light passingthrough the external light path window 23 becomes diffused. A rotatingshaft mounted at a motor is provided at the center of the switchingshutter 20, and a light path is selected by controlling the rotation ofthe switching shutter 20 with the motor.

In the apparatus achieved in the embodiment, the LD 7 is secured at afixed position and, the switching shutter 20 is rotated without varyingthe position of the light flux transmitted from the LD 7. However, FIG.15 shows the switching shutter 20 at a fixed position and the light fluxtaking on varying positions for purposes of facilitating theexplanation.

When the measurement light flux is to be irradiated on the target objectwhich is not a corner cube, the switching shutter 20 is controlled so asto set the light flux at a position 25A. The half of the light flux 25Athat is further toward the inside of the switching shutter 20 passesthrough the external light path window 22 and advances toward thesplitting prism 6. It is then reflected inside the dichroic prism 3 andis radiated toward the target object 9 via the focusing lens 2 and theobjective lens 1. The measurement light flux having passed the externallight path window 22 becomes condensed at the target object 9.

When a measurement is executed through the internal light path, theswitching shutter 20 is controlled so as to set the light flux at aposition 25B. The half of the light flux 25B that is further toward theinside of the switching shutter 20 becomes blocked by the switchingshutter 20, and thus, the outer half of the light flux 25B passesthrough the internal light path window 21, passes through the ND filter(not shown) and is reflected by an internal light path prism (not shown)before it is finally received at the APD 8. An explanation of ameasuring optical system for measuring the internal light path length,which is of the known art, is omitted.

When measuring a distance by using a corner cube as the target object,the switching shutter 20 is controlled so as to set the light flux at aposition 25C. The half of the light flux 25C that is further toward theoutside of the switching shutter 20 becomes blocked by the switchingshutter 20 while the other half is allowed to pass through the externallight path window 23. Since the diffusing member 24 similar to thediffusing plate 12 explained earlier is provided at the external lightpath window 23, the light flux emitted from the LD 7 becomes diffused.The diffused measurement light flux then advances to the splitting prism6, becomes reflected within the dichroic prism 3 and is then radiatedtoward the target object 9 via the focusing lens 2 and the objectivelens 1.

The distance measurement is performed while collimating and checking thetarget object through the collimating optical system. For this reason,the position of the focusing lens 2 is adjusted so as to allow lightpassing through the collimating optical system to be focused on thetarget object. However, due to the presence of the diffusing member 24,the focusing lens 2 and the collimating optical system no longer have aconjugate relationship to each other and, as a result, even though thecollimating optical system is in a focusing state in which light isfocused on the corner cube 10, the measurement light flux is defocusedand is not condensed on the corner cube 10. Since the measurement lightflux is not condensed on the corner cube 10 and thus the measurementlight flux is irradiated over a greater area, the reflected light fromthe corner cube 10 can be received at the APD 8 even if the center ofthe measurement light flux becomes offset from the center of the cornercube 10 due to a vibration at the corner cube 10 or the like. Thus,advantages similar to those realized in the first˜third embodiments areachieved through the distance measurement apparatus in the fourthembodiment.

While a diffusing member is provided at the switching shutter in thefourth embodiment as a deflecting optical member for changing the statein which the light is condensed, a member having a refracting functionmay be instead provided at the switching shutter. In addition, insteadof rotating the round switching shutter to switch from one light path toanother, the light paths may be switched by inserting/withdrawing theshutter.

In the fourth embodiment, an additional window is formed at theswitching shutter of the related art, which is provided to switchbetween the internal light path and the external light path, and adeflecting optical member is provided at the window. Thus, the desiredadvantages are achieved without having to add a new insertion/withdrawalmechanism.

Furthermore, the control of the switching shutter, i.e., the control ofthe insertion of the deflecting optical member at the light-transmittingoptical system, may be achieved through the method explained earlier inreference to the first and second embodiments.

While an explanation is given above on an example in which the presentinvention is adopted in a distance measurement performed by using pulselight, the present invention may be adopted in conjunction with otherdistance measurement methods as well. For instance, the presentinvention may be adopted in a distance measurement performed by usingthe difference between phases of modulated light.

What is claimed is:
 1. A focusing type distance measurement apparatuscomprising: a collimating optical system that collimates the light froma target object; a light-transmitting optical system that includes anobjective lens, a dichroic prism, a splitting prism and a laser diode,and that transmits a measurement light flux to the target object; afocusing optical system that includes a focusing lens and that condensesthe measurement light flux onto the target object; a light-receivingoptical system that receives a reflected light flux reflected from thetarget object via the focusing optical system; a deflecting opticalmember that changes a state in which the measurement light flux iscondensed on the target object; an operating member that outputs anoperation signal for performing distance measurement by using a prism asthe target object; and a control circuit that implements control on thedeflecting optical member to achieve a first state for the deflectingoptical member so as to change the state in which the measurement lightflux received at the light-receiving optical system is condensed whenthe following conditions are present: the operation signal has beenoutput through the operating member and a light reception levelindicating a quantity of the reflected light flux received at thelight-receiving optical system is equal to or lower than a predeterminedvalue or the light reception level manifests a significant change, andthat implements control on the deflecting optical member to achieve asecond state so as not to change the state in which the measurementlight flux received at the light-receiving optical system is condensedwhen the conditions are not present, wherein: the deflecting opticalmember includes a plurality of light transmittance adjustment areasprovided along a circumference of a rotating plate and a diffusion areawhere the light flux is diffused provided along a direction in which theplurality of light transmittance adjustment areas are arranged; thecontrol circuit includes a motor in order to rotate the rotating plate;and the diffusion area is inserted at a measurement light path when thedeflecting optical member is in the first state and one of the pluralityof light transmittance adjustment areas is inserted in the measurementlight path when the deflecting optical member is in the second state. 2.A focusing type distance measurement apparatus comprising: a collimatingoptical system that collimates the light from a target object; alight-transmitting optical system that includes an objective lens, adichroic prism, a splitting prism and a laser diode, and that transmitsa measurement light flux to the target object; a focusing optical systemthat includes a focusing lens and that condenses the measurement lightflux onto the target object; a light-receiving optical system thatreceives a reflected light flux reflected from the target object via thefocusing optical system; a deflecting optical member that changes astate in which the measurement light flux is condensed on the targetobject; an operating member that outputs an operation signal forperforming a short distance measurement by using a prism as the targetobject; and a control circuit that implements control on the deflectingoptical member to achieve a first state so as to change the state inwhich the measurement light flux received at the light-receiving opticalsystem is condensed when the operation signal has been output throughthe operating member and implements control on the deflecting opticalmember to achieve a second state so as not to change the state in whichthe measurement light flux received at the light-receiving opticalsystem is condensed when the operation signal has not been output,wherein: the deflecting optical member includes a plurality of lighttransmittance adjustment areas provided along a circumference of arotating plate and a diffusion area where the light flux is diffusedprovided along a direction in which the plurality of light transmittanceadjustment areas are arranged; the control circuit includes a motor inorder to rotate the rotating plate; and the diffusion area is insertedat a measurement light path when the deflecting optical member is in thefirst state and one of the plurality of light transmittance adjustmentareas is inserted in the measurement light path when the deflectingoptical member is in the second state.
 3. A focusing type distancemeasurement apparatus comprising: a collimating optical system thatcollimates the light from a target object; a light-transmitting opticalsystem that includes an objective lens, a dichroic prism, a splittingprism and a laser diode, and that transmits a measurement light flux tothe target object; a focusing optical system that includes a focusinglens and that condenses the measurement light flux onto the targetobject; a light-receiving optical system that receives a reflected lightflux reflected from the target object by condensing the reflected lightflux via the focusing optical system; a deflecting optical member thatchanges a state in which the measurement light flux received at thelight-receiving optical system is condensed; and a control circuit thatimplements control on the deflecting optical member to achieve a firststate for the deflecting optical member so as to change the state inwhich the reflected light flux is condensed when the following conditionis present: a light reception level indicating a quantity of thereflected light flux received at the light-receiving optical system isequal to or lower than a predetermined value or the light receptionlevel manifests a significant change, and that implements control on thedeflecting optical member to achieve a second state so as not to changethe state in which the reflected light flux is condensed when thecondition is not present, wherein: the deflecting optical memberincludes a plurality of light transmittance adjustment areas providedalong a circumference of a rotating plate and a diffusion area where thelight flux is diffused provided along a direction in which the pluralityof light transmittance adjustment areas are arranged; the controlcircuit includes a motor in order to rotate the rotating plate; and thediffusion area is inserted at a measurement light path when thedeflecting optical member is in the first state and one of the pluralityof light transmittance adjustment areas is inserted in the measurementlight path when the deflecting optical member is in the second state. 4.A focusing type distance measurement apparatus comprising: a collimatingoptical system that collimates the light from a target object; alight-transmitting optical system that includes an objective lens, adichroic prism, a splitting prism and a laser diode, and that transmitsa measurement light flux to the target object; a focusing optical systemthat includes a focusing lens and that condenses the measurement lightflux onto the target object; a light-receiving optical system thatreceives a reflected light flux reflected from the target object bycondensing the reflected light flux via the focusing optical system; adeflecting optical member that changes the state in which themeasurement light flux received at the light-receiving optical system iscondensed; an operating member that outputs an operation signal forperforming a short distance measurement by using a prism as the targetobject; and a control circuit that implements control on the deflectingoptical member to achieve a first state for the deflecting opticalmember so as to change the state in which the reflected light flux iscondensed when the operation signal has been output through theoperating member and implements control on the deflecting optical memberto achieve a second state so as not to change the state in which thereflected light flux is condensed when the operation signal has not beenoutput, wherein: the deflecting optical member includes a plurality oflight transmittance adjustment areas provided along a circumference of arotating plate and a diffusion area where the light flux is diffusedprovided along a direction in which the plurality of light transmittanceadjustment areas are arranged; the control circuit includes a motor inorder to rotate the rotating plate; and the diffusion area is positionedin a measurement light path when the deflecting optical member is in thefirst state and one of the plurality of light transmittance adjustmentareas is inserted in the measurement light path when the deflectingoptical member is in the second state.